Worse Than Usual, Without Failure

From an old apartment house, another one of the precursor takes on the concrete waffle slabs. The last one I discussed used precast concrete blocks to make up the room between the concrete ribs; this one used terra cotta blocks:

Structurally this works much like a modern waffle slab. Only the concrete ribs are expected to perform work carrying load. The terra cotta was put there to act as forms for the ribs when the concrete was wet and to act as furring to hold a plaster ceiling later on.

Of course, the focus of this picture isn’t the floor system. It’s the condition of the concrete. It is badly segregated – the coarse aggregate (gravel) is mostly at the bottom of the floor and the fine aggregate (sand) and cement are mostly at the top. There are some areas here that look more like gravel glued together than they do like concrete. We can see the rebar through the gaps in the concrete in the two ribs running up and down the page at the left and center.

That terrible concrete and the exposed rebar reduces the fire-rating of the floor, but that was probably not a dangerous condition before the plaster was removed during demo. More importantly, it reduces the force transfer between rebar and concrete that is the basis for the composite material we call “reinforced concrete.” If the concrete can’t transfer tension to the rebar, it’s not reinforced.

So…why hasn’t this fallen down yet? It’s likely that the floor has never seen code levels of live load, but the dead load is more than half of the total, so that’s not much help. More importantly, moment design of reinforced concrete in the 1920s was wildly over-conservative, badly undercounting the strength of both the concrete and the steel. So if you have a design that was twice as strong as needed to be, but construction quality defects reduced its capacity by half…you’re more or less okay. That doesn’t mean that these conditions can be left as is, and we patched the gaps to try to re-establish (or maybe establish for the first time) the bond between the rebar and concrete, but it means that, as awful as this looks, it wasn’t cause for panic.